In recent years, the trend toward micro-scale pattern rule has been increasing rapidly with the trend toward large-scale integration and high-speed of LSI. The trend toward a shorter wavelength of the exposure light source lies behind it. For example, it has become possible to mass-produce DRAM (dynamic random-access memory) of 64M-bits (processing dimension is 0.25 μm or less) by the wavelength shortening from mercury lamp i-line (365 nm) to KrF excimer laser (248 nm). Furthermore, in order to realize the production of DRAM's having integration degrees of 1 G or greater, a lithography using ArF excimer laser (193 nm) is used.
As a resist suitable for such exposure wavelength, “chemically amplified resist material” attracts much attention. This contains a radiosensitive acid generator (hereinafter referred to as “photoacid generator”), which generates an acid by radiation irradiation (hereinafter, referred to as “exposure”), and is a pattern-forming material that forms a pattern by making a difference in solubility between the exposed portion and the unexposed portion through a reaction using the acid generated by the exposure as a catalyst.
Various studies have also been conducted with respect to a photoacid generator used for such chemically amplified resist material. In case that a photoacid generator that generates an alkane or arenesulfonic acid, as used for chemically amplified resist materials, for which a conventional KrF excimer laser light is used as the light source, is used as a component of the above ArF chemically amplified resist materials, it is known that acid strength for severing an acid-labile group of the resin is not sufficient, resulting in no possibility of resolution at all, or it is known to be not suitable for the device production due to low sensitivity.
Therefore, as a photoacid generator of ArF chemically amplified resist materials, one that generates a perfluoroalkanesulfonic acid, which is high in acid strength, is generally used. Perfluorooctanesulfonic acid, or its derivatives are, however, known as PFOS by its initials, and stability (undegradability) and hydrophobicity resulting from C—F bond, and ecological concentration and accumulation resulting from oleophilicity have become problems. Furthermore, a perfluoroalkanesulfonic acid having a carbon number of 5 or greater or its derivatives also pose the above problems.
To deal with problems related to such PFOS, there is conducted by each place the development of a partially fluorine-substituted alkanesulfonic acid having a lowered fluorine substitution rate. For example, there have been the developments of alkoxycarbonylfluoromethanesulfonic acid onium salts as acid generators, such as triphenylsulfonium methoxycarbonyldifluoromethanesulfonato (Patent Publication 1), (4-methylphenyl)diphenylsulfonyl t-butoxycarbonyldifluoromethane sulfonato (Patent Publication 2), or triphenylsulfonium (adamantan-1-ylmethyl)oxycarbonyldifluoromethanesulfonato (Patent Publication 3).
On the other hand, there has been the development of triphenylsulfonium 1,1,3,3,3-pentafluoro-2-benzoyloxypropane-1-sulfonato or the like, which is one type of alkylcarbonyloxyalkane sulfonic acid onium salts, of which ester bond is opposite to that of the above-mentioned alkoxycarbonyldifluoromethanesulfonic acid onium salts (Patent Publication 4).
The present applicant has found a 2-alkylcarbonyloxy-1,1-difluoroethanesulfonic acid onium salt, in which the number of fluorines is less by three than that of the acid generator of Patent Publication 4, which is considered to be less in adverse effects on the environment, and has obtained findings that this substance functions as an acid generator having a strong acidity by the minimum number of fluorine atoms, is superior in terms of compatibility with solvent and resin, and is useful as an acid generator for resists (Patent Publication 5).
Furthermore, the present applicants have found a polymerizable tetrafluoroalkanesulfonic acid onium salt, which is a similar alkylcarbonyloxyalkanesulfonic acid onium salt, but the number of fluorines is less by one than that of the acid generator of Patent Publication 4, which is considered to be less in adverse effects on the environment (Patent Publication 6).
Here, as a method for synthesizing the polymerizable tetrafluoroalkanesulfonic acid onium salt of Patent Publication 6, there is disclosed a reaction pathway shown in the following reaction formula [1].
That is, it is a pathway equipped with a first step of obtaining a sulfinic acid metal salt by sulfinating 4-bromo-3,3,4,4-tetrafluorobutan-1-ol using a sulfinating agent, a second step of obtaining a sulfonic acid metal salt by oxidizing the obtained sulfinic acid metal salt using an oxidizing agent, a third step of obtaining a sulfonic acid onium salt by furthermore reacting the obtained sulfonic acid metal salt with a monovalent onium salt, and a fourth step of obtaining the target polymerizable sulfonic acid onium salt by reacting the obtained sulfonic acid onium salt with an alkylacrylic halide or alkylacrylic anhydride.
Furthermore, a similar tetrafluoroalkanesulfonic acid onium salt is disclosed in another publication, too (Patent Publication 7). In the publication, there is disclosed a synthesis method by using 1,4-dibromo-1,1,2,2-tetrafluorobutane as the starting material, and converting it into an aliphatic or aromatic carboxylic acid 4-bromo-3,3,4,4-tetrafluorobutyl ester by a selective substitution reaction using a carboxylate such as sodium carboxylate or ammonium carboxylate, and thereafter similar to Patent Publication 6 reacting the ester with a sulfine oxidizer such as sodium dithionite, in the presence of a base such as sodium hydrogencarbonate, in water, acetonitrile or a mixture thereof as a solvent to produce 4-acyloxy-1,1,2,2-tetrafluorobutanesulfinic acid salt, and then conducting an oxidization by a usual method by an oxidizer such as hydrogen peroxide solution in water as solvent in the presence of sodium tungstate.